Singlet exciton fission has attracted renewed interest in the last decade due to its potential to enhance power conversion efficiencies of single junction solar cells beyond the Shockley-Queisser Limit.1-6 The third-generation of solar cells is based on materials that operate by non-conventional photophysical mechanisms to overcome the Shockley-Queisser limit.7-9 The recent discovery of an efficient intramolecular singlet fission (iSF) process in conjugated polymers and small molecules has dramatically increased the quantity and variety of materials that exhibit this process.2 In molecules and polymers, singlet fission (SF) is the process whereby two triplets are generated from a single photon.17 The mechanism of triplet pair formation and decay may be quite different in dimers of oligoacenes relative to their monomer counterparts in the solid state, where singlet fission is an intermolecular process (xSF). For example, donor-acceptor polymers are presumed to undergo SF via charge transfer (CT) states, similar to the leading hypothesis for the mechanism for solid stateSF.10,18-20 However, there is no intrinsic CT character in molecular dimers, yet that have been reported to undergo SF at faster rates than the molecular systems.
Devices fabricated from singlet fission molecules have exceeded 100% external quantum efficiency,1,5 but many fundamental challenges remain: a) there are a limited number of materials that undergo SF; b) appropriate heterojunctions must be engineered to extract the multiple excitons; and c) device architectures that exploit SF must be engineered. While the resurgent interest in SF has been catalyzed by solar cells, multiexcitonic materials can be also widely applicable in other optoelectronic thin-film technologies.21 
Acenes are of great interest in the field of organic electronics due to their tunable optoelectronic properties, high charge carrier mobilities, and the observation of singlet exciton fission (SF) in crystals of tetracene and higher acenes.12,63-70 However, one major hurdle to the implementation of SF materials in devices is the need for intermolecular interactions. While various acenes have been shown to undergo quantitative intermolecular singlet fission (xSF), a significant impediment in these materials is that SF depends on intermolecular coupling. Such dependence on packing interactions prevents these materials from being widely applicable. For example, acenes can undergo SF only when neighboring chromophores are appropriately coupled by a charge-transfer (CT) state in crystalline solids or highly concentrated solutions—intermolecular singlet fission (xSF).5, 20,24 However, practical applications of xSF materials are difficult because the process relies on intermolecular coupling between chromophores that depend sensitively on crystalline structure and film morphology. A more suitable approach that would enable high-throughput screening of materials is to employ intramolecular singlet fission (iSF) active layers. iSF has rarely been observed in organic materials, with yields lower than 30% or as an endothermic process.25,26 
The design of organic materials based on strong-donor/strong-acceptor copolymers and small molecules that facilitate iSF through a photoexcited state with strong CT-character, exhibit up to 170% triplet yield in a polymer.10 Such design principles were founded on the CT-mediated mechanism of xSF. Interestingly, there is another strategy in molecular materials that stems from the fundamental concept of xSF, which involves the covalent coupling of two SF chromophores. To date, several groups have attempted to model or synthesize dimers for iSF, but experimental triplet yields have been low (<10%).2,3,27-31 
Molecular dimers made up of two covalently linked SF-capable monomers have been considered as candidates for iSF28,32. However, early work on tetracene dimers showed low iSF yields, presumably because of the endothermicity of the iSF process or the connectivity employed.29-30 Pentacene dimers, on the other hand, have recently been reported to undergo iSF quantitatively.13,15 Pentacene is of particular interest, as it is a benchmark material for organic field effect transistors (OFETs) and organic photovoltaics (OPVs), as well as fundamental studies of various optoelectronic properties.71-76 However, pentacene has only limited stability and solubility in common organic solvents and is unstable in the presence of oxygen,47 making it difficult to process by high throughput techniques.65,77,78 To overcome these limitations, several functionalized pentacenes have been reported, which exhibit enhanced solubility, stability, and tunable electronic properties.72,79,80 Despite these improvements, over the course of nearly 80 years of significant research in pentacene chemistry and physics, there has been only one report of short conjugated oligomers, a scarce number of conjugated pentacene-containing polymers, and a pentacene homopolymer remains unknown.81-86 The potential to develop families of oligoacene dimers through systematic studies provides motivation to revisit the concept of singlet fission in oligoacene “mixtures”, which was briefly explored in the 1970s when several groups studied crystals of one type of acene doped that had been doped with another type of acene.32-34 
There are various important aspects that are still being actively investigated in terms of electronic structure, excited state energies and dynamics.22,35-37 Thus it is important to elucidate the mechanistic and energetic requirements for iSF in order to optimize the design of practical SF chromophores. Therefore, there is still a need for simple, stable, and soluble molecules that exhibit quantitative singlet fission, preferably, for example, molecules that undergo ultrafast iSF. Such fast iSF combined with triplet pair lifetimes as long as hundreds of picoseconds may enable harvesting of two triplets or two electron hole pairs for devices with enhanced photocurrents.